Flyingron is right…. Speed trap laws apply only when speed is determined by electronic means. To my knowledge, this has not been extended to car speedometers (which clearly are electronic devices), but conversely, to my knowledge, electronic speedometers have not been excluded either. It is certainly worth a try and it clearly falls within the black letter of the law:
40802. (a) A "speed trap" is either of the following:
…if that prima facie speed limit is not justified by an engineering
and traffic survey conducted within five years prior to the date of
the alleged violation, and enforcement of the speed limit involves
the use of radar or any other electronic device that measures the
speed of moving objects.
Personally, I would think that it would be harder to argue that electronic speedometers are NOT within the definition of the law than to argue that they are. The statement “any other electronic device that measures the speed of moving objects” is pretty clear. I’m not sure how one could say that a speedometer does not measure the speed of a moving object.
In order to follow this defense, I would send a discovery request to the DA and ask for, among other things, the make, model and year of the police cruiser (as well as the speedometer calibration records… for which there will be none). If you get the car information, it should be simple to look up some parts information to show that the speedometer is truly an electronic device.
Second possible defense is that you could argue that the prosecution had not provided speedometer calibration records. While it is generally accepted that a speedometer is a reasonably accurate device, this assumption is predicated on a constant tire size. Various tire sizes will significantly change the speedometer reading for the same speed. For an extreme example, two tire sizes: 395/95R16 vs 395/20R16. These are both 16 inch tires and it is POSSIBLE to use them on a 16 inch wheel (which is reasonable to assume the wheel size for the cruiser is). At 40mph, the speed difference between the two tires is 19.5mph!! That is HUGE!! This is easily obtained by doing a bit of math, or you can use the calculator at http://tire-size-conversion.com/tire-size-calculator/ . So, since the officer will have testified prior to you saying anything, you should first ask him if he used the speedometer in making the determination of the speed of his car and your car. Once he says “yes”, then you should move to strike his testimony that was based on his speedometer reading because the prosecution had not introduced into evidence the size of tire used on the patrol car at the time of the incident as compared to the original stock tire size provided by the manufacturer. Your argument should not be that you are disproving the accuracy of the officer’s speedometer, but rather the prosecution has failed to provide prima facie evidence necessary for the court to rely on the officer’s speedometer.
This is likely to fail. Not because it is not a very valid and legally sufficient argument… but more likely because the judge will be pissed that you raise such an obvious point in court. If the judge overrules your motion to strike, you should object. You must make the record clear that you have objected to the introduction of speedometer data and the judge must overrule you so that the issue will be preserved for appeal.
For a third defense… this is a bit more esoteric and will require a bit more skill in arguing… but here goes… In People v DiFiore, the court stated:
“A person who drives in excess of the maximum lawful speed has not been subjected to a "speed trap" even if his speed has been detected by radar on a posted road unless the officer relies on the posted or prima facie speed limit. The officer who does rely on the posted speed limit improperly benefits from the anti-speed-trap laws, if that officer is permitted to testify to a speed that is both unlawful and excessive, but the posted speed is not justified by a traffic survey.”
In other words, the officer cannot say that since you exceeded the posted speed limit, your speed was unsafe while providing no evidence that the speed limit is the accepted “safe speed”. Typically, officers who pace are not subject to speed trap laws due to the fact that they are in traffic at the time and they can rely on first hand information about the safety of a driver’s speed. So, if the officer ONLY presents evidence that your speed was unsafe (and a violation of the basic speed law) simply because you exceeded the posted speed limit, then he is effectively stating that he did NOT use his first hand perspective to evaluate the safety of your speed based on the conditions at the time. Instead, he will be relying solely on the posted speed limit as defining a safe speed. Your argument will then be that the posted speed limit is not justified and therefore it is improper for the officer to rely on that as the only evidence of your speed being unsafe. To be clear… this is not calling the arrest a “speed trap”. This is saying that the officer must prove that your speed was unsafe as per the basic speed law and that and unjustified speed limit in itself does not do that.
So… here are three viable defenses. Each of them have significant legal merit and could be used to prevail in this case if skillfully argued. But they will require you to do some legwork and become very familiar with the issues presented here. I have only scratched the surface. This will require some work on your part. You may not want to go through the effort, or you may not have confidence in your ability. That’s ok. However, you shouldn’t be discouraged by those who will follow this post trying to poke holes in these defenses unless they can provide valid legal reasons for why they won’t work (something other than an emotional response).
Good luck. Post questions if you have them.

